1. Field of the Invention
The present invention relates to a piston operable to force frictional coupling elements of a clutch or brake device provided in an automatic transmission, against each other to place the clutch or brake device in its engaged state.
2. Discussion of the Related Art
A known automatic transmission includes clutches and brakes each of which is provided with frictional coupling elements in the form of a plurality of friction plates, so that the automatic transmission is automatically shifted by selectively engaging and releasing the clutches and brakes. The frictional coupling elements are forced against each other by a piston, to place each clutch or brake in its engaged state. JP-9-32919A and JP-10-89380A disclose an example of such a piston, which includes two piston members, namely, a first piston member forming a cylindrical portion of the piston, and a second piston member forming a bottom portion of the piston. In the piston disclosed in those publications, the first piston member has axial cutouts formed in one of its opposite axial end portions, while the second piston member has engaging jaws fitted in the axial cutouts, and a retainer ring is fitted in a circumferential groove formed in the inner circumferential surface of the first piston member, in contact with the second piston member, so that the first and second piston members are fixed to each other into the piston. The circumferential groove consists of a plurality of arcuate grooves that are spaced apart from each other by the axial cutouts in the circumferential direction of the first piston member.
In the piston disclosed in JP-9-32919A, an axial movement of the piston causes a load to be applied from the second piston member to the circumferential groove of the first piston member through the retainer ring. The engaging formed on the second piston member extend in the radial direction of the piston and are abuttable onto the retainer ring, so that the load is applied from the engaging jaws to the retainer ring upon the axial movement of the piston. The retainer ring is merely fitted in the circumferential groove of the first piston member, in contact with the second piston member having the engaging jaws fitted in the axial cutouts of the first piston member, without any member supporting the retainer ring on its side remote from the engaging jaws, the application of the load from the second piston member to the circumferential groove of the first piston member through the retainer ring causes a stress concentration on the portions of the circumferential groove that are adjacent to the axial cutouts in the circumferential direction of the first piston member. Accordingly, those portions of the circumferential groove tend to elastically deform due to the stress concentration.
To prevent the elastic deformation of the circumferential groove of the first piston member in the piston of JP-9-32919A, therefore, the retainer ring must have a sufficiently large thickness, undesirably resulting in an increase in the size of the piston.